1. Field of the Invention
The present invention relates to an antenna unit using a helical antenna employed mainly in a radio paging receiver such as a pager and a radio receiver device using the same.
2. Description of the Related Art
In the radio paging receiver such as the pager in the prior art, normally the loop antenna is used as the antenna unit. A configuration of the antenna tuning control circuit in the loop antenna for the pager in the prior art will be explained with reference to FIGS. 13 and 14.
As shown in FIG. 13, the antenna tuning control circuit is constructed such that the impedance matching circuit 102 is connected to the antenna 101, the central processing unit (CPU) 104 is connected to the control input terminal of the impedance matching circuit 102 via the low pass filter (LPF) 103, and the high frequency amplifier 105 and the mixer 106 are connected to the output terminal of the impedance matching circuit 102. The receiving frequency of the antenna 101 and the impedance matching circuit 102 can be set by inputting the control voltage from the CPU 104 after noise components are removed from the control voltage by the LPF 103. The LPF 103 consists of the resistor R1 and the capacitor C1. The received signal being set at the receiving frequency as above is amplified by the high frequency amplifier 105 and then set out to the receiver circuit including the mixer 106 at the succeeding stage.
The loop antenna employed as the antenna 101 is made up of antenna material such as metal, etc. which has a very short loop length (length about 1/5.lambda. to 1/10.lambda., e.g., the loop length of about 15 cm to 10 cm when the receiving frequency band is set to 280 MHz) relative to the wavelength. Therefore, since the loop antenna has a very narrow frequency bandwidth, the frequency setting is needed every receiving frequency and thus the impedance matching is performed by the impedance matching circuit 102 each time.
FIG. 14 shows a concrete configuration of the antenna 101 and the impedance matching circuit 102 shown in FIG. 13. The antenna 101 is composed of antennas L1, L2 which are formed by a circuit pattern whose equivalent circuit is given by two series-connected coils, etc. The impedance matching circuit 102 is constructed such that a variable capacitor (trimmer capacitor) TVC and a variable capacitance diode (varactor diode) DVC are connected in series, and the variable capacitance diode DVC is grounded via a resistor R2 and also grounded via a capacitor C2 to pass a high frequency component. These antennas L1, L2 are connected in parallel with the variable capacitor TVC and the variable capacitance diode DVC.
The variable capacitor TVC is provided to absorb variation of inductance of the antenna. The resonance frequency of the antennas L1, L2 can be changed by varying the capacitance to be set to the receiving frequency f0. The variable capacitance diode DVC is provided to change the receiving frequency to another receiving frequency. The receiving frequency can be set by applying the control voltage from the CPU 104 every channel. It is possible to change the receiving frequency by adjusting the impedance in this manner.
The loop antenna has the problem that, if such loop antenna is located away from the human body, reduction in the antenna gain of about 5 dB is caused at the frequency of the 280 MHz band, for example, because of lost of the body effect. In recent, applications of the radio paging receiver are broadened other than the application as the so-called "calling terminal". For example, there are applications as the information receiving terminal, the equipment controlling receiver terminal, etc. If the radio paging receiver is utilized as the information receiving terminal or the equipment controlling receiver terminal, it is often employed in a free space other than the neighborhood of the human body. In this case, improvement of the antenna gain due to the body effect cannot be expected. In addition, a null point is generated in the antenna radiation pattern since directivity of the loop antenna exhibits the 8-shaped characteristic, so that there is the case where the sufficient antenna characteristic cannot be guaranteed as the receiver.
According to a configuration of the above example in the prior art, in order to set the impedance at the receiving frequency f0, the impedance matching circuit which consists of variable capacitance elements such as the variable capacitor TVC, the variable capacitance diode DVC, etc. is needed to thus increase the configuration of the device in size. This is disadvantageous from the viewpoints of miniaturization and cost reduction by reducing the parts packaging area. In addition, in order to change the receiving frequency f0 into another receiving frequency, the receiving frequency switching circuit which can set the channel by applying the control voltage from the CPU to the variable capacitance diode DVC every change of the channel is needed. Therefore, there is the problem that both the configuration of the device and its operation become complicated. In particular, if the antenna unit is separated from the receiver to use as an external antenna unit, a control line for applying an antenna control voltage from the CPU in addition to the receiving signal transmission line is also needed. Therefore, there is caused the problem that a cable wiring becomes complicated.
In the case that the whip antenna is employed as the external antenna unit, the large size antenna which has a vertical length of 25 cm, even if the 1/4.lambda. antenna is employed, for example, is required at the 280 MHz band, and ideally the whip antenna is installed on the ground plate (referred to as a "GND plate" hereinafter) such as the iron plate, etc. as the installing location. Therefore, there are the problems that not only the size of the device is increased but also the installing location of the antenna unit is restricted.